One striking characteristic of blood coagulation is the fine level of control placed on the process. While functioning normally, blood should not clot, and likewise, when it is appropriate, blood should clot quickly and firmly. We are interested in determining the crystallographic structures of several of the molecules that keep clotting under control (factor XIII-inhibitor complexes, corn Hageman factor inhibitor, heparin co-factor II). While we have solved the structure of the factor XIII zymogen, we now have small crystals of these complexes and molecules that will require the use of synchrotron radiation to complete their structure analyses,